Face seal assemblies are employed in gas turbine engines to prevent leakage of fluid along the engine's rotating shaft where the shaft extends through a wall or partition. These assemblies are comprised of a rotating component called a seal rotor and a non-rotating component called a face seal. The face seal is usually lightly spring loaded against the seal rotor.
Historically, various materials have been used for both the seal rotor and the face seal. For example, metals, carbon, ceramics, and other materials are mentioned in Zobens, U.S. Pat. No. 4,174,844, Floyd et al., U.S. Pat. No. 4,036,505, Fenerty et al., U.S. Pat. No. 3,926,443, and Stahl, U.S. Pat. No. 3,770,181. A common configuration is to have a metallic seal rotor and a carbon face seal. A problem with these seals is that oil coking results from the friction between the seal rotor and the face seal. Also, the carbon face seal tends to wear which requires that the engine be removed from service regularly to either inspect or replace the seal.
It is well known by those skilled in the art, that a carbon face seal will wear at a lower rate when rubbing against a ceramic surface as opposed to a metallic surface. Accordingly, one proposal for increasing the life of a conventional face seal assembly is to replace the metallic seal rotor with a ceramic seal rotor, (see for example Fenerty et al., teaching a seal assembly for a water pump in which one of the seal rings is ceramic, column 1, lines 50-55). However, such technology is not applicable to gas turbine engines because the rotating components in these engines are assembled in a lockup. This means that the rotating components, (the compressor disks and turbine disks including the seal rotors) are first stacked one atop the other and then forced, and held together by a large compressive force. This compressive force produces concentrated tensile stresses on the sealing surfaces of the seal rotors abutting a rotating component. Because of its brittle nature conventional ceramic seal rotors tend to crack under this compressive force.
Accordingly, a need still exists for a face seal assembly, for a gas turbine engine, having a seal rotor that can withstand the compressive force of a lockup assembly and also provides a ceramic surface for sealingly engaging a carbon face seal.